n=int(input())
if not(999<n<10000):
    print(0)
    exit()
n=str(n)
sp=[]
n1, n2, n3, n4 = n[0], n[1], n[2], n[3]
a=[n1, n2, n3, n4, n1+n2, n2+n3, n3+n4, n1+n2+n3, n2+n3+n4, n1+n2+n3+n4]
for i in a:
    fl=True
    i=int(i)
    for j in range(2, i):
        if i % j==0:
            fl=False
            break
    if fl==1:
        sp.append(i)
if len(sp)==0:
    print(0)
    exit()
else:
    print(max(sp))




